The invention relates to the cleaning and passivation of the copper products employed in tape automated bonding (TAB) in semiconductor device packaging. Here a copper tape is provided with a series of metal finger patterns along its length. The fingers in each pattern extend inwardly to form an array that mates with the bonding pad array on a semiconductor device such as an integrated circuit (IC). In the earliest approach the IC's were treated while in wafer form to provide raised or bumped bonding pads to which flat copper fingers are TC bonded. The bumped bonding pads typically included an upper layer of copper usually located over a nickel barrier layer. The bumped bonding pads act to stand the metal fingers off of the chip surface so that short circuits are avoided. In a later developed system the bonding pads are kept flat and relatively thin. The meta tape finger ends incorporate raised bumps that can be thermocompression (TC) bonded to the flat bonding pads on the IC. All of the pads can be bonded to the metal tape fingers simultaneously in a gang bonding operation. Once the IC pads have been bonded to the tape fingers, in what is called an inner lead bonding (ILB) operation, the chip becomes mechanically attached to the tape and is transported and handled therewith in subsequent operations.
It is important that the copper bumps and/or tape be cleaned to eliminate oxides that can interfere with the bonding operation both of ILB and in the subsequent bonding of the attached fingers to some other structure. Typically, the copper is cleaned by treating it with a sulhuric acid pickling bath. This treatment actually removes a thin surface layer of copper thereby to expose a new clean surfavr. This is a drastic treatment and leaves a copper surface that is fully exposed to the atmosphere which can result in further oxidation. That is, the copper is not passivated.
U.S. Pat. No. 3,806,366 relates to the continuous pickling of cast copper rod stock. As the rod emerges from a rolling mill it is hot and is desirably quenched before further treatment. The quench can be produced using a pickling bath that includes a mild organic acid such as citric acid. Thus, the pickling process makes use of the heat from the rod in order to accelerate the pickling operation. The sudden cooling of the rod stock causes it to contract and thereby burst off the oxides from the surface and expose it to the mild acid.
U.S. Pat. No. 2,318,559 relates to a copper pickling bath that contains ferric salts and a relatively small amount or organic acid. The object is to produce a smooth, bright and natural appearing copper surface.
U.S. Pat. No. 4,188,438, which is assigned to the assignee of the present invention, covers the concept of providing the copper surfaces in a TAB process with a corrosion resistant coating that is thin enough so that the coated surface can be TC bonded through the coating. This reference also teaches the various elements of TAB using the bumped bonding pad approach
U.S. Pat. No. 4,005,472, which is also assigned to the assignee of the present invention, shows the coating of the wafer bumps with electroless gold in a thin layer after the bumps have been cleaned to remove any surface oxides.
The teaching in the above citations is incorporated herein by reference.